1. Field of the Invention
The present invention relates to power control circuits, and in particular to a timed switching circuit for reducing the average voltage applied to a load.
2. Description of Related Art
With the increasing costs of energy, the demand for energy saving devices is easily justified economically. Electrical power conversion is performed routinely but still is relatively inefficient. In addition power conversion is often not performed because of these inefficiencies, when conversion might otherwise seem appropriate. For example, voltage saturation often occurs with appliances powered by house current, because this voltage is often greater than need be. This mismatch may constitute a waste of 25% to 35% of useful power.
Power supply voltages have been transformed in two ways: a ferromagnetic transformer or by a sine wave switching. Solid state devices such as triacs or silicon control rectifiers (generically referred to as thyristors) facilitate sine wave modification. Still there is a need for a more efficient techniques to increase the general efficiency of power usage.
Known dimmer circuits vary the conduction angle through a thyristor to establish a desired drive to a load. These circuits typically provide adjustment up to 100% conduction. While these known dimmer circuits employ thyristors, they have not been designed to supply a stable, reduced, secondary voltage with a static conduction angle.
Such conventional dimmer circuits have been marketed as energy conservation devices wherein reducing the drive to a load such as a lamp can produce modest to large savings. (See for example Energi-Saver dimmer by Energy Saver, Center Valley, Pa.). These known dimmers however, have not provided special features for reducing power while still maintaining a high level of brightness and service. See also NASA Motor-Control Circuit, Popular Electronics, Oct. 1979, page 39.
Other known circuits employing silicon controlled rectifiers have employed restive capacitive circuit in parallel with the with the thyristors to facilitate commutation. Without proper circuitry to facilitate such commutation, certain loads (e.g., inductive loads) can produce transients that adversely affect the conduction of the thyristor and may prevent it from turning off at the end of a half cycle. F. W. Gutzwiller, Silicon Controlled Rectifier Manual, at 183-187 (1967).
Other known thyristor switching circuits have dealt with cycle skipping, that is, a failure to ever conduct during a half cycle as intended. For example, known circuits have employed unijunction transistors with a resistor-capacitor timing circuit for triggering the unijunction. There is a risk of the capacitor in the unijunction timing circuit not fully discharging at the end of a half cycle. If the capacitor does not fully discharge, then the unijunction may not fire later and may skip one or more half cycles. Known solutions to the cycle skipping problem are employing resistors in the appropriate position in the unijunction circuit at the proper value.
Also, unijunction triggers for activating thyristors can at times be sensitive to power transients. One known technique for immunizing the unijunction transistor is to connect a bootstrap capacitor between base 2 (the higher voltage base) and the emitter of the unijunction transistor. Id. at 338.
Another disadvantage with such known converters is the inability to handle high power. Known circuits have employed silicon controlled rectifiers connected in parallel. The gates of these controlled rectifiers have been triggered by a pilot silicon controlled rectifier. This arrangement allows a pilot rectifier of modest power rating to control a large bank of rectifiers, thereby increasing the effective power handling capacity. Id. at 131
A known voltage converter (U.S. Pat. No. 3,430,101) employs a thyristor switch that reduces the conduction angle of current through an incandescent lamp. The converter works with house current whose voltage magnitude is rated at 120 volts, 60 Hertz. By reducing the conduction angle, this relatively high voltage can drive a low voltage lamp rated at 12 volts. A disadvantage with this known converter is that it is a three wire system: one wire to a supply line, a second wire to the other supply line and one terminal of the lamp, and the third wire to the other terminal of the lamp. Thus if such a circuit were mounted at a wall switch, three wires must be routed from the switch box, instead of the usual two. See also U.S. Pat. Nos. 3,358,186; 3,493,848; 3,525,015; and 3,684,919.